Photoelectric tube



April 1938. F. M. PENNING ET AL 2,112,975

PHOTOELECTRIG TUBE I Filed April 3, 1937 v INVENTORS mm'MPmu/m MARIENQTEVES BY 3 Mo IIENDRIK LEM; l

W ATTORNEY Patented Apr. 5, 1938 Frans Michel Penning, Marten Cornelis Teves, and Hendrik Lems, Eindhoven, Netherlands,

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assignors to N. .V. Philips Gloeilampenfabrie- Application April 3, 1937, Serial No. 134,802

In Germany April 6, 1936 Y 3 Claims. (Cl. 250-5275) This invention relates toa photo-electric tube having an electrode which is impervious to the active rays and which comprises an adsorbed film of photo-electric metal, an excess of free photo-electric metal not being present in the tube. The active light rays which cause the photo-electric electrode to emit electrons enter the tube through a window in the tube wall.

It has been found that the inside wall of the window of the tube often sets up or accumulates charges which cause erratic operation of the tube and is evidenced by irregularities in the characteristic curve of the tube.

According to this invention, this difficulty is obviated by coating the inside wall of the en-' velope at the window with a coherent metal film which is in conductive connection with one of the electrodes and is thin enough to transmit most of the active light rays. The metal film is a continuous coherentjlayer of metal.

The thicker the'fihn on the window, the lower will be its electric resistance but of course will be the greater the absorption of light. By a few simple tests, the minimum permissible value of resistance of the film may be determined necessary to prevent the accumulation of wall charges. Generally, the light absorption may be arranged to be less than 50%, and only 10' or 20% of light absorption will preferably be allowed. Thus, for example, it has been found that silver films which are thin enough to absorb less than 20% of the transmitted light have sufficient conductivity to avoid the irregularities in the characteristic curve.

The film on the window may be formed of a metal, such as silver volatilized from a filamentary electrode, or from a metal having a particularly high melting point (over 1500 C.) for example platinum, tungsten or molybdenum, since it is possible to form silver films which even in layers of minute thickness have sufficient conductivity and low light absorption.

If one of the electrodes is arranged on the inside of the tube wall, as in the case with many kinds, the film on the window may be conveniently connected to this electrode.

In order that the invention may be clearly understood and readily carried into effect it will now be described more fully with reference to the accompanying drawing, which illustrates by way of example a photo-electric tube according to the invention.

In the drawing, the tube envelope I is shown essentially spherical and is provided with a tubular side stem piece 2 having a reentrant stem silver layer 1.

and pinch 3 through 'whichare sealed the leading-in wires 4 and 5 of the wire-shapedfilament electrode 6. In operation electrode 6 may function asan anode; This electrode may be formed of tungsten wire which prior to its introduction into the tube is coated with silver. After ex haustion of the tube, this silver is thermallyvolatilized by the passage of an electric current through the tungsten wire. The volatilized silver becomes deposited on the inside of the spherical partof the wall, where it forms a silver film A screen 8 secured to' the wire Binsures that the window-9 is not coated with silver and-the screen l0 prevents the silver deposit from extending intothe part 2. The quantity- 0f s'ilver which is volatilized may be such that the silver layer 1 has a thickness between about 0 .01 and 0.1 millimeter.

After the formation of thes'ilver layer Ta small quantity of rare gasysuch as argon at a pressure of 0.1 millimeter, is introduced into the tube and in-this gaseous atmosphere a glow discharge is struck between the silver layer 1 as the cathode and electrode 6 as the anode, the discharge being of such intensity that silver particles are disintegrated from the layer 1 and deposited on the inside of the window 9 where the particles form a thin transparent conducting silver film H in conductive connection with the The disintegration may be continued until the layer II has a thickness of between 0.005 and 0.01 micron. The argon is then removed from the tube and oxygen at a pressure of about 0.1 millimeter is introduced into the tube, whereupon a glow discharge is struck in the oxygen between the silver layer 1 as the cathode and the electrode 6 as the anode. The current strength and the duration of this glow discharge are so chosen that part of the silver layer 1 is oxidized to form a silver oxide film on the silver.

After the oxidation the remaining oxygen is removed from the tube and a photo-electric metal, for example caesium or another alkaline or alkaline earth metal, is introduced into the tube. Caesium may be introduced into the tube in any known manner, such as by a distillation process or by liberation of the active metal within the tube from a mixture of a caesium compound and a reducing agent. Caesium becomes deposited on the silver oxide layer and meanwhile the tube is subjected to a temperature of about C., the tube being preferably not in open communication with the vacuum pump, to reduce the silver oxide by the caesium. The $11- ver layer 1 then has a film l2 comprising a mixture of silver oxide particles, silver particles and caesium particles. In addition, a thin caesium film is adsorbed by this mixed layer. The excess of caesium may be pumped away, or removed by a substance which combines with the caesium, such as lead oxide, or removed by a metal, such as tin, which alloys with caesium.

Although the thin silver layer ll may be partly converted into an oxide while the silver layer 1 is being oxidized, there is no affect on the action of the layer II. If oxidation of the silver layer II is desired to be avoided, this layer may be formed subsequent to the oxidation of the silver layer 1 by volatilizing a small quantity ofsilver in the tube after the said oxidation.

It has been found that the irregularities in the characteristic curves of the tube are obviated by .the conducting layer II on the window and that when the tube is provided with a gaseous atmosphere the breakdown voltage of the tube is more constant than in the absence of the layer ll.

The conducting layer H may be formed of metals other than silver. For example, the inside of the wall of the spherical part of the tube may be entirely coated or coated at least at the area where the window is provided with a thin layer of platinum or molybdenum, for example by volatilization in vacuo, from a filament of platinum. Then the silver layer that constitutes the substratum of the photo-electric cathode may be applied as above described. For the purpose of obtaining a sufiiciently conducting layer ll of platinum or molybdenum, the thickness of the layer may be considerably less than that of silver, which reduces light absorption. Further, during the oxidation of the silver layer, the layer H would be oxidized to a less extent.

Since many modifications may be made in this invention without departing from the spirit of this invention, it is desired that this invention be limited only by the prior art and the appended claims.

We claim:

1. A photo-electric device comprising an envelope of light pervious material, a coating of light impervious metal on a portion of the interior wall of said envelope, light responsive electron emitting material on said coating, and a layer of metal pervious to light and with good electrical conductivity in conducting relation to said metal coating on another portion of said envelope, and an electron collecting electrode in said envelope supported in insulated spaced relation to said coating.

2. The process of manufacturing a photo-electric device with an envelope interiorly coated with metal containing light responsive electron emitting material, comprising depositing a light impervious coating of said metal over a portion of the interior wall of the envelope leaving an uncoated portion as a window for the admission of light, depositing an electrical conducting layer of metal on the window in contact with the metal coating and thinner than said metal coating and which is pervious to light, and photo-electrically activating said metal coating.

3. The process of manufacturing a photo-electric device with an envelope interiorly coated with a layer of metal containing light responsive electron emitting material comprising depositing said metal over a portion of the interior wall of the envelope leaving an uncoated portion as a window for the admission of light, striking a glow discharge to the metal coating todisintegrate part of the metal and deposit it in a thin light pervious layer on the window portion of the envelope, and photo-electrically activating said metal coating.

FRANS MICHEL PENNING. MARTEN CORNELIS 'I'EVES. HENDRIK LEMS. 

